1. Technical Field
The present invention relates to communication networks and, more particularly, to wireless communication networks.
2. Related Art
Initially, communication systems, and more particularly, wireless communication systems, would route calls through the actual trunks that carry the calls. Stated differently, the trunks would be used to establish the link from the calling party to the called party without knowing the availability of the called party. The impact of this approach was that a trunk would be tied up for call routing purposes even if the call was not eventually connected. While the telephone resources would be released as soon as it was clear that the called party was not available, the wasteful allocation of resources for calls that could not be connected, in aggregate, resulted in significant inefficiencies.
Accordingly, telecommunication networks evolved to include signaling systems to facilitate the routing of call setup signals without having to tie up actual communication trunks. More generally, the purpose of a signaling system is to transfer control information between elements in a telecommunications network. The elements may comprise switches, operation centers and databases. For example, in some telecommunication networks, the elements comprise base station transceiver sets (BTSs), base station controllers (BSCs), mobile switching centers (MSCs), home location registers (HLRs), etc. Thus, signaling systems were originally developed to establish connections between telephone offices and the customer premises equipment (namely, the telephones) in order to transport voice traffic through a voice-oriented telephone network. To explain in simple terms, the signaling systems use a separate channel for conducting signaling control messages.
FIG. 1 shows a prior art signaling network. The signaling network, shown generally at 10, comprises BS 14 controlled by MSC 18 and BS 22 controlled by MSC 26. MSC 18 and MSC 26 communicate with home location (HLR) 30 through gateway MSC (G-MSC) 34 to access mobile station information to establish a communication link between mobile stations located within a service area served by BS 14 and BS 22 for establishing calls to and from mobile stations 38 and 42. Logically, MSC and G-MSC functions can be separated. In practice these are typically the same physical device that performs both functions within a mobile network.
In operation, a mobile station, such as mobile station 38, generates call setup signals through a BTS to a base station controller (BSC), collectively BS 14, in this example, which then generates calling signals to MSC 18. For the present example, assume that mobile station 38 is placing a call to mobile station 42. Neither BS 14 nor MSC 18 know what BS is serving mobile station 42 or what MSC is serving mobile station 42. Accordingly, MSC 18 produces call setup signals to a G-MSC 34 which queries a home location register (HLR) 30 to determine a location of mobile station 42. More specifically, HLR 30 returns an identifying MSC, which here is MSC 26, to G-MSC 34.
In some cellular networks, HLR 30 would merely communicate with the destination MSC as a part of the call setup procedures. Accordingly, in those embodiments, HLR 30 may communicate with MSC 26 to advise it that MSC 18 is attempting to establish a call to a mobile station served by MSC 26, as well as identifying information to enable MSCs 18 and 26 to communicate to setup the trunk for the voice traffic. In other networks, for example, in global systems for mobile communications (GSM) networks, the G-MSC, here G-MSC 34, communicates with HLR 30 to determine a destination MSC. Accordingly, in these embodiments, HLR 30 returns the identity of MSC 26 to G-MSC 34 which, in turn, forwards the response to MSC 18. The MSC 18 then communicates with MSC 26 to further set up the call. In the example shown, MSC 18 communicates with MSC 26 by way of G-MSC 34. Accordingly, the call connection may be established for a mobile station 38 through each of the communication elements to mobile station 42.
As the dual networks evolved for carrying user traffic and signaling traffic, the focus of many designs was to also provide redundancy, not only for the user traffic, but also for the signaling links. Thus, it is common to have a redundant signaling link, perhaps in a different cable bundle or, in a fiber optic ring, routed in an opposite direction, to provide an ability for control signaling to propagate through the network to establish a call despite a link or element failure. Heretofore, however, no geographically separated redundancy has been provided for the endpoints of the signaling network that carries the signaling traffic. More particularly, in phone networks, and especially in cellular phone networks, no geographically separated redundancy has been provided for the mobile switching centers and the gateway mobile switching centers.
Typically, the MSCs are consolidated at a switching center geographically distant from the cellular networks they control. One problem associated with consolidating the switching function is a geographically localized event, such as a tornado or other natural/man-made disaster will disrupt communications over a large area for any network element for which geographically separated redundancy is not provided.
The control or signaling network includes switching and transport of signaling streams and is separated from the processing of the call sessions that control the bearer streams. The switching and transport of the bearer streams is executed by media gateways and packet switches in the switching and transport layer of the network. The processing of call sessions that control the bearer streams is executed by call servers in the call control layer of the network. The switching and transport layer of the network typically uses asynchronous transfer mode (ATM) or Internet protocol (IP) technology.
One advantage of having bearer-independent core networks is that call servers can handle calls for media gateways that are distributed across a very large geographic area. One shortcoming of this approach, however, is that the risk resulting from the failure of a call server due to a power outage or natural disaster at the call server site, as mentioned above, affects communication services, and more particularly, wireless communication services, over a larger geographic area than the failure of a traditional cellular mobile switching center. Accordingly, what is needed is a call server that may be coupled and formed to act as a backup server in the event of a failure of a primary call server even if the backup call server is geographically dispersed from the primary call server.